1. Field of the Invention
This invention relates to a method for fabricating a cathode ray tube having a high degree of fineness and, more particularly, to an improved method for obtaining a luminance and a colorimetric purity for a cathode ray tube panel.
2. Description of the Prior Art
In order to form images with a high fineness in cathode ray tubes, it is necessary to form a fluorescent layer having a very fine pattern of a distinctly striped or dotted form on a panel surface without any separation. For accurate landing of an electron beam on such a fine pattern, a beam spot has to be converged as small as possible. To supplement the shortage of the luminance, techniques of improving the luminance of the panel surface are essential.
In recent years, however, as the fineness of the panel surface is remarkably improved, it is becoming more difficult to deposit and keep fluorescent particles in an optimum amount in a desired position. This is because when the fluorescent layer containing fluorescent particles is formed in a fine pattern, its adhesion to the panel surface is lowered so that the fluorescent layer is liable to come off at the time of development. Such coming off or removal will cause the color balance to be lost and the luminance to be lowered. It may be considered that if the fluorescent layer is made thin, the layer is prevented from coming off, but it has a lower luminance. Accordingly, there is a strong demand for techniques of effectively preventing the fluorescent layer from coming off when the layer is formed with a relatively large thickness. However, no effective measure has yet been proposed.
On the other hand, several techniques have been proposed for improving the luminance of the panel surface irrespective of the prevention of the fluorescent layer from coming off or being removed.
One such typical example is a metal backing. This is a technique of forming an aluminum thin film having a high light reflectance and a high electron transmittance on the fluorescent layer, for example, according to a vacuum deposition method. Among fluorescence rays which the fluorescent material emits by energization with an electron beam, the component directed toward the back face is reflected in the forward direction to improve the brightness of the picture. Besides, the metal backing has functions of preventing an ion spot and stabilizing the potential of the fluorescent face.
In the metal backing step, prior to the vapor deposition of the aluminum film, a thermally decomposable intermediate film, such as of nitro cellulose, polymethacrylate, acrylic emulsions or the like, is smoothly formed. This intermediate film is removed by decomposition during a subsequent thermal treatment, leaving an aluminum film alone on the inner surface of the panel. In order to improve the luminance of the cathode ray tube, it is essential; to form the aluminum thin film in a state of a mirror surface. To this end, the intermediate layer is formed with a relatively large thickness to absorb the surface irregularities on the fluorescent layer. Alternatively, for the formation of the intermediate film, the inner surface of the panel has water applied thereto, after which a lacquer is developed thereon.
Another approach for improving the luminance of the panel surface is a technique wherein a light reflection layer having a high light reflectance is formed on the light-absorbing matrix, such as by a vacuum deposition, an application of slurry or the like. The present invention defines a light-absorbing matrix as a pattern of a striped or dotted form on a panel surface.
For instance, Japanese Patent Publication No. 63-29374 discloses a technique of depositing a nickel thin film by an electroless plating of nickel-phosphorus selectively on a carbon matrix. In a cathode ray tube Of the type where a fluorescent layer is formed to extend onto the carbon matrix according to a so-called inside exposure method, the nickel thin film can prevent light emitted from the fluorescent particles from being absorbed in the carbon matrix, thereby improving the luminance and the contrast ratio on the panel surface.
Further, similar results are obtained by a technique disclosed in Japanese Patent Publication No. 63-40011, wherein a suspension containing both a light-absorbing material, such as graphite, and a light diffusion and reflection material, such as titanium oxide, is applied on the inner surface of the panel and developed to form a light diffusion and reflection layer on the carbon matrix.
3. Problems to be Solved by the Invention
However, in the prior art techniques, many problems are left to be solved.
First, although the prior techniques show effects to an extent with respect to the improvement in the luminance, the coming-off problem of the fluorescent layer is not solved by any existing technique. More particularly, the light reflection layer formed by plating, vacuum depositing or applying slurries has so smooth a surface that high adhesion to the fluorescent layer cannot be expected. In addition, the technique of forming the light reflection layer by electroless nickel-phosphorus plating is complicated in the formation step.
Second, the improvement of the luminance by the prior art techniques is limited and, especially, the problem of the lowering of the colorimetric purity in the color cathode ray tube has not been solved. This is a problem having a relation to the formation of the fluorescent layer.
The formation of the fluorescent layer can be broadly classified into an inside exposure method wherein light is exposed from the side of the inner surface of the panel and an outside exposure method wherein light is exposed from the outer side of the panel as proposed by the same applicants in Japanese Laid-Open Patent Application No. 60-119055.
In general, in the inside exposure method, the light exposure for forming the fluorescent layer is performed from the inside of the inner surface of a panel glass, so that, as shown in FIG. 5, part of a fluorescent layer 25 is partially extended over a carbon matrix 22. Accordingly, there is, for example, a luminous ray component which is emitted from the fluorescent particles and is directly absorbed in the carbon matrix 22, as shown by arrow l.sub.11 in FIG. 5, or another luminous ray component, as shown by arrow l.sub.12, which is reflected from a metal backing film 26 and is then absorbed in the carbon matrix 22. Thus, there exists a problem that the luminance is not improved even though the amount of the fluorescent particles is large.
According to the technique of the Japanese Patent Publication No. 63-29374, the lowering of the luminance can be suppressed to an extent when a thin nickel film is deposited on the carbon matrix. However, since the fluorescent layers are formed by the inside exposure method and the distance between adjacent fluorescent layers becomes small, a luminous ray emitted from a given fluorescent particle passes straightly, as shown by an arrow l.sub.13, in FIG. 5 or is reflected by the metal backing film 26, as shown by arrow l.sub.14, with the possibility of entering an adjacent fluorescent layer 25'. With color cathode ray tubes, adjacent fluorescent layers 25, 25' contain fluorescent particles of different colors from each other and, thus, the passing of rays between the layers 25 and 25' will cause the colorimetric purity to be lowered. The thin nickel film does not have such a height as to establish a partition wall between the fluorescent layers 25, 25', so that luminous rays, as shown by the arrows l.sub.13 and l.sub.14 cannot be effectively shielded or stopped.
To cope with the problem of the color mixing, there has been proposed a deposition of a thin nickel film only on marginal portions of the carbon matrix 22. In this arrangement, a light-absorbing effect at the central portion of the carbon matrix 22 is expected with the luminance-improving effect being reduced.
The outside exposure method is a technique of enabling one to form self-aligned fluorescent layers through a carbon matrix mask by exposing light from the outside of the panel. According to the outside exposure method, the fluorescent layer 24 is not extended over the carbon matrix 22, as shown in FIG. 4, but is selectively formed on non-formed portion or window portions of the carbon matrix 22. Thus, the uniformity and colorimetric purity can be remarkably improved when this method is applied to color cathode ray tubes.
However, the loss of the luminance cannot be avoided even with the use of the outside exposure method for the following reason. In the outside exposure method, sharp steps are formed between the regions where the thick fluorescent layer 24 has been formed and the regions where the thin carbon matrix 22 has been formed. If, for example, an acrylic emulsion is used to form an intermediate layer (not shown), the emulsion is liable to flow toward a recess (i.e. the carbon matrix). The resultant intermediate film is formed with an incline, which is inevitably provided on a metal backing layer 26 formed thereon. Accordingly, among luminous rays emitted from fluorescent particles, there is a component which is reflected at the inclined face 26a of the metal backing film 26, as shown by arrow l.sub.10, and this reflected component is finally absorbed in the carbon matrix 22 to lead to a loss of the luminance.
The common problem involved in the inside exposure method and the outside exposure method is that a so-called aluminum lifting, i.e., the separation of the metal backing, takes place. This is because, as stated above, the intermediate film is liable to be formed as a thick portion on the carbon matrix and an amount of the gases generated from the thick portion during the thermal decomposition step becomes so large that an additional pressure of the gases is exerted on the metal backing film. The separation results in a lowering of the luminance. In order to avoid the partial difference in the thickness of the intermediate film, the intermediate film may be formed entirely as a thick film. This will contribute to an increase in the degree of the mirror surface of the metal backing film, but will also increase an amount of the gases from the decomposition and unfavorably facilitate the aluminum lifting.
Thus, it is very difficult in the prior techniques to simultaneously achieve the prevention of both the fluorescent layer from coming off and the aluminum lifting and the improvements in the luminance and the colorimetric purity.